Dwarf plants have had a major impact on agriculture. Dwarf varieties of wheat are widely used in North America due to both reduced potential for lodging and high yields. Dwarf fruit trees are also extensively used and allow farmers to produce more fruit per acre thereby increasing economic yield potential. There are other benefits that may be realized from the use of dwarf crop plants and dwarf fruit trees including reductions in the amounts of pesticides and fertilizers required, higher planting densities and reduced labor costs.
In view of the current trends of both increasing human population and the decreasing land area suitable for agriculture, increasing agricultural productivity is, and will continue to be, a challenge of paramount importance. Dwarf crop plants and fruit trees have been and will continue to be important components of our agricultural production system. Increased usage of dwarf crop plants and dwarf fruit trees may help to meet the agricultural production demands of the future. However, commercially acceptable dwarf varieties are not available for all crops.
In addition to the use of dwarf plants to control plant height, synthetic chemicals are routinely applied to certain economically important plant species to reduce growth. Plant growth regulators known as growth retardants are used to reduce stem elongation in a variety of crops including cotton, grape vines, fruit trees, peanuts, wheat and ornamentals such as azaleas, chrysanthemums, hydrangeas, poinsettias and many bedding plants. All of the commonly used growth retardants are inhibitors of gibberellin biosynthesis and limit stem or shoot growth by reducing elongation. In the United States, the most widely used growth retardant is mepiquat chloride, which is registered for use on cotton. Benefits attributed to the use of mepiquat chloride on cotton include increased yield, improved defoliation, improved stress tolerance, more uniform crop maturity and the ability to harvest earlier. Previously, the growth retardant daminozide was registered for use in the United States on apples, grapes and peanuts under the trademarks ALAR and KYLAR but was removed from use on food crops due to human health concerns. Despite the demands of agricultural producers for a product to replace diaminozide, there are no growth retardants registered for use on grapes, fruit trees and peanuts in the United States. Daminozide, however, is still widely used on certain non-food, plant species.
Uncovering the molecular mechanisms that control plant growth processes such as cell division and cell elongation will likely aid in the development of new plant varieties with reduced stature and new methods for reducing plant growth. Such new plant varieties and methods may provide both farmers and horticulturists with environmentally benign alternatives to the use of synthetic growth-retarding chemicals.
Elongation of plant cells and organs is one of the most critical parameters of plant growth and development. Regulation of this trait in plants, however, is a fairly complicated process, as both external and internal factors influence it. The most important external stimulus is light, with its normally repressible or negative effect on cell elongation (Quail, P. H. (1995) Science 268:675-680; Kende et al. (1997) Plant Cell 9:1197-1210). The internal control of cell elongation is mediated by a number of chemicals, normally referred to as plant growth regulators or hormones (Kende et al. (1997) Plant Cell 9:1197-1210). Among the classical plant hormones, auxins and gibberellins (GAs) both promote cell elongation whereas cytokinins and abscisic acid each have been shown to have a negative effect on cell elongation (Kende et al. (1997) Plant Cell 9:1197-1210). Recently, another class of plant growth regulators, named brassinosteroids, has been identified that also dramatically promote plant growth (Yokota, T. (1997) Trends Plant Sci. 2:137-143; Azpiroz et al. (1998) Plant Cell 10:219-230; Choe et al. (1998) Plant Cell 10:231-243). However, the mechanisms by which plant hormones act, either singly or in concert, to control cell elongation remains unclear.
One way to gain an understanding of mechanisms that mediate cell elongation is to study mutants in which this aspect of plant growth is compromised (Klee et al. (1991) Annu. Rev. Plant Physiol. Plant Mol. Biol. 42:529-551). Numerous such mutants have been identified across most plant species, including maize, in which more than 25 single-gene mutations that affect plant stature have been characterized (Coe et al. (1988) In: Corn & Corn Improvement, G. F. Sprague (Ed.) Madison, Wis.; Sheridan, W. F. (1988) Annu. Rev. Genet. 22:353-385). These dwarf mutants are considered to be GA related, mainly because GA is the only phytohormone whose role in regulating height in maize has been convincingly established (Phinney et al. (1985) Curr. Top. Plant Biochem. Physiol. 4:67-74; Fujioka et al. (1988) Proc. Natl. Acad. Sci. USA 85:9031-9035). Both types of mutants, GA responsive and GA non-responsive, have been found in this collection of maize mutants. While genes for a number of GA-responsive mutants have been cloned and found to be involved in GA biosynthesis (Bensen et al. (1995) Plant Cell 7:75-84; Winkler et al. (1995) Plant Cell 7:1307-1317), nothing is known about the nature of defects in GA non-responsive maize mutants.
One type of GA non-responsive dwarf mutants that have received much attention from maize geneticists and breeders is called brachytic. These dwarfs are characterized by internodes of substantially reduced length, relative to wild-type, without having any effect on the size or number of other organs, including the leaves, ear and tassel (Kempton, J. H. (1920) J Hered. 11:111-115). There are three known brachytic mutations in maize, br1, br2 and br3, all of which are recessive (Coe et al. (1988) In: Corn & Corn Improvement, G. F. Sprague (Ed.) Madison, Wis.; Sheridan, W. F. (1988) Annu. Rev. Genet. 22:353-385). Because of the commercial interest in br2 for enhancing plant productivity (Pendleton et al. (1961) Crop Sci. 1:433-435; Duvick, D. N. (1977) Maydica 22:187-196; Djisbar et al. (1987) Maydica 32:107-123; Russel, W. A. (1991) Adv. Agron. 46:245-298), this dwarf has been characterized the most. Depending on the genetic background, plants homozygous recessive for br2 are 30-70% shorter than their normal sibs. This reduction in plant height is exclusively due to a reduction of the length of stalk (stem) internodes. In addition to being dwarf, br2 mutants grown under greenhouse conditions often suffer from buggy whip, a disease-like condition in which the unfurling leaves in the whorl undergo necrosis and stay stuck together. This condition often results in the death of the growing tip of the plant.
Although the dwarfing trait in maize has been extensively studied both genetically and molecularly, it has yet to be exploited successfully in breeding efforts in this crop plant. In contrast, dwarf mutants of sorghum have contributed significantly to the development of modern day cultivars. Sorghum and maize are both members of the grass (Poaceae or Gramineae) family and thus share many characteristics including genomic organization and plant body form. Out of the four dwarfing mutations exploited in sorghum, dw3, whose dwarfing phenotype looks very similar to that of br2 in maize, appears to be the most prominent. However, the only dw3 allele (dw3-ref) available thus far has a serious problem which limits its agronomic value. The dwarf phenotype associated with the dw3 allele is unstable, with a reversion frequency to wild-type (tall) as high as about 1% in certain genetic backgrounds. The instability of this dwarf phenotype, the mechanism of which has eluded sorghum geneticists thus far, not only continues to embarrass sorghum breeders, but also sometimes leads to the rejection of an otherwise promising inbred or hybrid.
To keep up with the demand for increased agricultural production, new targets are needed for genetically engineering agricultural plants for the improvement of agronomic characteristics. Elucidating the molecular mechanisms of cell division and elongation will provide new targets for agricultural scientists to manipulate.